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Difficulty: MediumIgnition System

As an internal combustion engine accelerates to higher operating speeds (RPM), what adjustment must the ignition system make to spark timing to ensure maximum combustion pressure develops at the optimal point in the power stroke?

  1. A
    Increase electrical output from the alternator to accelerate the flame front speed across the combustion chamber
  2. Advance the spark timing so ignition occurs earlier before top dead center (TDC)Answer
  3. C
    Retard the spark timing so ignition occurs during the exhaust stroke to burn residual gases
  4. D
    Enrich the air-fuel mixture so combustion occurs instantaneously without modifying spark timing

Answer

Advance the spark timing so ignition occurs earlier before top dead center (TDC)
Because the air-fuel mixture requires a finite and relatively constant duration to burn completely, firing the spark earlier before top dead center (advancing timing) at higher engine speeds ensures that peak cylinder pressure is reached just as the piston begins descending on the power stroke.

Step-by-Step Solution

1
Analyze the relationship between engine rotational speed and fuel combustion duration
As engine RPM increases, the piston completes its strokes in less time, while the air-fuel charge still requires approximately 2 to 3 milliseconds to reach peak combustion pressure.
Understanding that mechanical piston velocity changes with RPM while chemical flame propagation remains relatively constant is essential to spark timing.
2
Determine the necessary ignition timing correction
The ignition system must fire the spark plug earlier in the compression stroke (more degrees before top dead center, or advanced) so peak combustion pressure aligns with the start of the power stroke.
Advancing spark timing maintains optimal thermal efficiency and engine torque across varying RPM ranges.

Key Concept

Ignition timing advance compensates for increased piston speed at higher RPM by triggering the spark earlier before top dead center.
Estimated Time:1m 0s
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Topics

Ignition System

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